As the development of the wind turbine technology, the height of a wind turbine generator system is increasing. Currently, the height of the new type of wind turbine generator system can reach and even exceed 160 meters. However, the wind turbine generator system is typically mounted on high altitude terrains or seaboard regions which are relatively open and tends to suffer from lightning strikes as a prominent object within these areas for their height. It is necessary to provide a lightning protection device for a wind turbine generator system so that the wind turbine generator system can be prevented from being damaged by the lightning strikes.
Currently, a Permanent Magnet Direct-Driven (PMDD) wind turbine generator system is usually protected against damages due to lightning strikes with the following method. The entire PMDD wind turbine generator system uses metal casting parts, structural parts and bearings of its own as the conducting paths, except that separate cable wires used inside of the blades. Specifically, the down lead of the blade arrester is connected directly to the flange at the roots of the blades of the PMDD wind turbine generator system; the flange is connected to the hub via the pitch bearing; the hub is connected to the generator via the main rotor bearing of the generator; the generator is connected to the base frame of the nacelle which is connected to the tower by the yaw bearing; and the tower is connected to the ground; thus, the lightning current can be conducted into ground.
However, according to the method mentioned-above, the lightning current will certainly flow through the pitch bearing, the main shaft bearing and the yaw bearing of the PMDD wind turbine generator system. Because of the gaps in the bearings, the impedance of the path through which the lightning current passes is high, which influences the discharging of the lightning current. Consequently, there is a chance that the bearings would be burned and thus damaged by the lightning current. However, the operations of replacing the bearings of the wind turbine generator system are complicated and the total cost of maintenance will be increased. Further, bearing damage will result in operation interruption of the wind turbine generator system, which may also result in huge economical loss and maintenance cost.